Grand Unification has been the holy grail of physics for about a century. It is the belief that all forces of Nature are in reality different manifestations of the same force; that there is a deep unifying simplicity underlying the natural laws that we would be able to see if we could only unlock the key principles at work. There is good reason to believe that this belief is not an unrealistic one.
By the mid 1800's, physicists had achieved a decent understanding of three forces prevalent in the world around them: gravity, electricity, and magnetism. Gravity was the earliest to be discovered and the most familiar one. Magnets had also been studied extensively by that time and were known to be sources of some mysterious non-gravitational force. And electricity had just been discovered through a series of experiments. Static electricity - responsible for the shock you get when you grab a door knob after petting a particular fluffy cat - had been identified even centuries before.
In the late 1800's, a dramatic development occurred in theoretical physics: a physicist by the name of James Maxwell demonstrated on paper that the electric and magnetic forces are really the same force, the "electromagnetic force". They can simply be related by changing your perspective: if you just move around with respect to an electric force, you will see a magnetic force as well… The significance of this development was two-fold: it was the first time that we realized that Nature can fool us by appearing more complex than it actually is; and it was the first time that an entirely theoretical and conceptual process lead us to new physics. These two novelties were to become permanent themes in physics from then on.
In the early 1900's, two more forces of Nature were to be discovered: the "weak force" and the "strong force". Both ruled the world at very small distances - where quantum mechanics takes over. Their discovery had to be preceded with some understanding of the crazy quantum world first. The weak force is associated with radioactivity, while the strong force is responsible for nuclear power. And by the mid-1900's, theoretical physicists realized that all four of the known forces - gravity, electromagnetism, the weak force, and the strong force - are related to a series of profound symmetries within the natural laws, the so-called gauge symmetries (see previous post for more). But the four forces still looked very different. Can the success of uniting the electric and magnetic forces of the mid-1800's be replicated once again?
In the 1970's, another dramatic development demonstrated that this was indeed the case. A couple of theoretical physicists managed to show that the electromagnetic and weak forces are actually the same force law in disguise, the "electroweak force": one down, three to go. Their proposal involved the prediction of a new particle, the Higgs particle (see post on the God particle). This particle is yet to be discovered (however see post on the LHC), but the circumstantial evidence for the correctness of the electroweak theory has been so overwhelming that the authors of the work were quickly awarded the Nobel prize.
So, we're down to three forces: gravity, the electroweak force, and the strong force. In recent years, we have learned that it is indeed very possible to unite the electroweak and strong forces as well - we call these frameworks GUTs (Grand Unified Theories). However, this program has many directions, as well as its share of problems. Only with more experimental data can it get pinned down definitively. But conceptually, there should not be any serious obstacles preventing us from uniting the electroweak and strong forces; unlike the case of their sister force…gravity.
So, that leaves the oldest force law, the gravitational force, the orphan of the story. Unfortunately, this last step of unifying all the forces of Nature is a major one: it involves resolving serious inconsistencies between gravity and quantum mechanics. To date, the best known candidate theory we have to address this issue comes in the form of String Theory (see previous post for a bit about this subject; more to come in due time…). It is however the case that testing this benchmark experimentally may come either centuries into our future or tomorrow… It feels like we are in striking distance of Grand Unification, yet the last hurdle is indeed a humongous one.
Accompanying this post are two video excerpts, parts of a longer documentary that explores this narrative. It's a total of 25 minutes of video for both, but the presentation content is well done and includes interviews with some of the most interesting theoretical physicists of our time - including Steven Weinberg, a co-author of the electroweak unification work.